Unraveling the mechanisms of S-doped carbon nitride for photocatalytic oxygen reduction to H2O2

First-principles calculations reveal the mechanisms of thermodynamically feasible process for the selective oxygen photoreduction to H2O2 in the S-doped melon-based carbon nitride.

The drug ornidazole inhibits photosynthesis in a different mechanism described for protozoa and anaerobic bacteria

Ornidazole of the 5-nitroimidazole drug family is used to treat protozoan and anaerobic bacterial infections via a mechanism that involves preactivation by reduction of the nitro group, and production of toxic derivatives and radicals. Metronidazole, another drug family member, has been suggested to affect photosynthesis by draining electrons from the electron carrier ferredoxin, thus inhibiting NADP+ reduction and stimulating radical and peroxide production. Here we show, however, that ornidazole inhibits photosynthesis via a different mechanism. While having a minute effect on the photosynthetic electron transport and oxygen photoreduction, ornidazole hinders the activity of two Calvin cycle enzymes, triose-phosphate isomerase (TPI) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Modeling of ornidazole's interaction with ferredoxin of the protozoan Trichomonas suggests efficient electron tunneling from the iron–sulfur cluster to the nitro group of the drug. A similar docking site of ornidazole at the plant-type ferredoxin does not exist, and the best simulated alternative does not support such efficient tunneling. Notably, TPI was inhibited by ornidazole in the dark or when electron transport was blocked by dichloromethyl diphenylurea, indicating that this inhibition was unrelated to the electron transport machinery. Although TPI and GAPDH isoenzymes are involved in glycolysis and gluconeogenesis, ornidazole's effect on respiration of photoautotrophs is moderate, thus raising its value as an efficient inhibitor of photosynthesis. The scarcity of Calvin cycle inhibitors capable of penetrating cell membranes emphasizes on the value of ornidazole for studying the regulation of this cycle.

Oxygen photoreduction and its effect on CO2 accumulation and assimilation in air-grown cells of Synechococcus UTEX 625

Mass spectrometric measurements of 16O2, 18O2, and 13CO2 were used to measure the rates of gross O2 evolution, O2 uptake, and CO2 assimilation in relation to light intensity, temperature, pH, and O2 concentration by air-grown cells of the cyanobacterium Synechococcus UTEX 625. CO2 fixation and O2 photoreduction increased with increased light intensity and, although CO2 fixation was saturated at 250 μmol ∙ m−2 ∙ s−1, O2 photoreduction was not saturated until about 550 μmol ∙ m−2 ∙ s−1. At high light intensity addition of inorganic carbon to the cells stimulated O2 photoreduction 2-fold when CO2, fixation was allowed and 5-fold when CO2, fixation was inhibited with iodoacetamide. The ability of O2, to act as an acceptor of photosynthetically generated reducing power was dependent upon the O2 concentration, and the substrate concentration required for half maximum rate (K½(O2)) was 53.2 ± 4.2 μM (mean ± SD, n = 3). The Q10 for oxygen photoreduction was about 2. A certain amount (10%) of O2 appeared to be required for maximum photosynthesis, as photosynthesis was inhibited under anaerobic conditions, especially at high light intensity. The point of inhibition is unknown but it seemed unlikely to be on CO2 transport or the concentration of intracellular dissolved inorganic carbon (Ci), as the rate of initial CO2 transport was enhanced and the intracellular Q1 pool increased in size under anaerobic conditions. Key words: cyanobacteria, photosynthesis, Ci concentrating mechanism, inorganic carbon pool, O2 photoreduction, electron transport, temperature.

The effects of inorganic carbon and oxygen on fluorescence in the cyanobacterium Synechococcus UTEX 625

The active transport of inorganic carbon and the accumulation of the internal pool caused quenching of chlorophyll a fluorescence both when CO2 fixation was allowed or when CO2 fixation was inhibited. Upon the addition of inorganic carbon in the presence of 240 μM oxygen the rate of change in fluorescence (or quenching) was correlated (r = 0.98) with the rate of active CO2 uptake, and the extent of quenching was correlated (r = 0.99) with the size of the internal inorganic carbon pool. Fluorescence was quenched by the fixation of inorganic carbon in the absence of oxygen but the reoxidation of QA following a flash of light was slow. In the presence of inorganic carbon, with or without the inhibition of CO2 fixation, oxygen quenched fluorescence. If CO2 fixation was inhibited, the degree of quenching depended upon the oxygen concentration with a K1/2 (O2) of about 42 μM. Below 60 μM oxygen there was a further reduction of QA following a flash of light and the reoxidation of QA was slow. Rapid reoxidation of QA following a flash of light required about 240 μM oxygen. From the response to added 3-(3,4-dichlorophenyl)-1, 1-dimethylurea, the quenching by oxygen was photochemical quenching and nonphotochemical quenching did not seem to be present. For reasons that are unknown, however, only about 80% of the quenching could be reversed with high intensity flashes of light. The photoreduction of oxygen was regulated by the presence of inorganic carbon, although fixation of CO2 was not required. The mechanism of this regulation is not known but it may be due to bicarbonate relief of electron transfer between QA and QB. Some results on measuring Fo, F′o, Fm, and F′m, in Synechococcus UTEX 625 are presented. Key words: cyanobacteria, fluorescence, oxygen photoreduction, active inorganic carbon transport.